Skin-core structure and bimodal Weibull distribution of the strength of carbon fibers

The bimodal Weibull parameters of polyacrylonitrile-based carbon fibers were obtained from the fiber-bundle tension test. The bimodality increases with increasing high temperature treatment up to 2100 °C and then decreases for the highest temperature treatment of 2400 °C. The local structure of the fibers was investigated by scanning X-ray microdiffraction using synchrotron radiation. This enables the mapping of the atomistic structure across the fiber with a position resolution of 100 nm. The mechanical bimodality is attributed to a reversal of the observed skin structure from less orientation to higher orientation as compared to the core.